1. Field of the Invention
This invention relates to electronic signal modulation, and more specifically, to spread spectrum modulation of binary messages.
2. Description of Related Art
Many systems provide for a plurality of transmitter and receiver pairs, "communicating pairs", to communicate simultaneously with little or no interference between communicating pairs. This may be accomplished by allocating a different communication frequency to each communicating pair, known as frequency division multiple access (FDMA). There are situations in which it is difficult or impossible to assign each communicating pair a unique frequency. This occurs when there are simply too many communicating pairs close to each other on a designated bandwidth.
Another access method, such as time division multiple access (TDMA), may be employed, wherein each communicating unit is assigned a "time slice" in which to communicate on the same frequency band. This causes the "time slices" to become shorter as the number of communicating pairs increases.
Since both FDMA and TDMA communicate on narrowly defined bands, they are both susceptible to narrowband interference. Another method employs a spreading each message from each communicating pair across the entire usable bandwidth. They are all specially encoded such that they may be separated at the receiver. These are known as spread spectrum techniques.
Spread spectrum techniques are those modulation techniques which require a transmission bandwidth that far exceeds the message information bandwidth. The spread spectrum modulation characteristics should not depend upon the individual message to be transmitted as is the case with some other wideband modulation schemes such as wideband FM. There are many spread spectrum mechanisms. They can be conveniently classified as: (i) direct sequence, (ii) frequency hopping, (iii) time hopping, and (iv) hybrids.
In direct sequence spread spectrum modulation, a wideband carrier signal is combined with the relatively narrowband message to yield an encoded wideband signal. A typical digital implementation would be to create a high speed random binary sequence in having an equal probability of being a one or zero at any particular time. This high speed binary sequence is added to a binary message sequence. The addition is typically done by exclusive-ORing ("XOR") the two sequences together. The bits of the message sequence are much longer in duration than the bits from the high speed random source and thus many random bits are used per information bit. The random bits are often referred to as "chips" and the relationship between the random sequence rate and the message rate is such that an integral number of chips are used per message bit. The code used to "spread" the signal at the transmit unit, is required in "despreading" the signal at the receive unit.
There are many uses for direct sequence spread spectrum techniques. One primary use is that of spectrum sharing. It is possible for a number of different communicating pairs to occupy the same bandwidth simultaneously without significant mutual interference. This is usually accomplished in direct sequence spread spectrum systems by assigning each communicating pair a different spectrum spreading code. This is known in the art as code division multiple access (CDMA).
Since the messages are spread over a large bandwidth, there is less possibility of loss of communications due to narrow bandwidth interference. The effect of the interference is therefore reduced by the bandwidth over which it is applied.
Typically, one problem with direct sequence spread spectrum communications and CDMA is synchronization. In order to function properly, the receiver must generate a signal from the spreading code which is the inverse from the transmitted spread spectrum signal, and do so at the same rate as the transmit unit. The receive unit must also correct relative phase discrepancies between the transmit and receive unit. This requires synchronization to be established and maintained at a tolerance finer than a single chip width. Further, the receiver must know the particular spreading code employed by the transmit unit in order to decode the signal and recover the message.
Currently there is a need for a simplified method of direct spread spectrum modulation which is not as sensitive as conventional systems to synchronization, and which does not require the spreading code in advance.